On-Chip Generation of Co-Polarized and Spectrally Separable Photon Pairs

TL;DR

Spectrally pure heralded photons are essential for scalable on-chip quantum photonics but SPDC sources typically exhibit intrinsic spectral correlations that limit purity and interference visibility. The authors demonstrate a co-polarized SSPP source in thin-film LiNbO3 that uses higher-order TE modes to achieve group-velocity matching under type-0 phase matching, complemented by Gaussian-apodized poling and an on-chip mode converter to route photons into separate channels. The device yields a nearly factorable joint spectral amplitude with a joint spectral intensity purity around 94% and a heralded purity around 89%, with mode-conversion efficiency exceeding 95%. This co-polarized dispersion engineering enables flexible spectral tailoring within a single polarization, reducing circuit complexity and supporting scalable quantum computing and networking applications.

Abstract

On-chip generation of high-purity single photons is essential for scalable photonic quantum technologies. Spontaneous parametric down-conversion (SPDC) is widely used to generate photon pairs for heralded single-photon sources, but intrinsic spectral correlations of the pairs often limit the purity and interference visibility of the heralded photons. Existing approaches to suppress these correlations rely on narrowband spectral filtering, which introduces loss, or exploiting different polarizations, which complicates on-chip integration. Here, we demonstrate a new strategy for generating spectrally separable photon pairs in thin-film lithium niobate nanophotonic circuits by harnessing higher-order spatial modes, with all interacting fields residing in the same polarization. Spectral separability is achieved by engineering group-velocity matching using higher-order transverse-electric modes, combined with a Gaussian-apodized poling profile to further suppress residual correlations inherent to standard periodic poling. Subsequent on-chip mode conversion with efficiency exceeding 95\% maps the higher-order mode to the fundamental mode and routes the photons into distinct output channels. The resulting heralded photons exhibit spectral purities exceeding 94\% inferred from joint-spectral intensity and 89\% from unheralded $g^{(2)}$ measurement. This approach enables flexible spectral and temporal engineering of on-chip quantum light sources for quantum computing and quantum networking.

Figures (4)

• Figure 1: Design principle and simulation of the co-polarized spectrally separable photon pair (SSPP) source. (a) Simulated group indices and corresponding optical mode profiles of the pump (785 nm, TE$_0$), signal (1520 nm, TE$_0$), and idler (1620 nm, TE$_2$) in the TFLN waveguide, illustrating the modal configuration used in this work. Scale bar: 500 nm. (b) Schematic of the integrated device incorporating the Gaussian-apodized poling region and the on-chip mode converter. (c) Target PMF showing the balance between spectral purity and effective nonlinear strength (brightness). (d) Simulated JSA with periodic poling, showing a sinc-shaped PMF with residual spectral correlations in the sidelobes. (e) Simulated JSA with Gaussian-apodized poling, yielding a Gaussian spectrum with suppressed sidelobes. (f) Schematic of the Gaussian-apodized domain inversions (left) and SHG microscope image confirming the realized domain inversions (right). Scale bar: $5~\mu\mathrm{m}$.
• Figure 2: Experimental characterization of the PMF via SFG process. (a) Schematic of the measurement setup. Two tunable cw telecom lasers are coupled into the device to generate SFG light. One beam is converted into a higher-order mode through on-chip mode converter, while the other remains in the fundamental mode. The generated SFG signal is collected and detected using a silicon photodetector, which is insensitive to the input telecom wavelengths. (b) Measured SFG mapping showing a positively sloped phase-matching response with suppressed sidelobes, in agreement with simulations.
• Figure 3: Joint-spectral characterization of the co-polarized SSPP. (a) Experimental setup for spectral shaping of the pump light and coupling it into the device. A 4-f grating filter with a photomask at the Fourier plane sets a 4.5-nm bandwidth at a center wavelength of 784 nm. (b) Schematic of the JSI measurement based on a fiber-dispersion spectrometer. The generated signal and idler photons are separated, coupled into two 40-km single-mode fibers for frequency-to-time mapping, and detected by SNSPDs using TCSPC. (c) Reconstructed JSI of the biphoton state, exhibiting a Gaussian profile with strongly suppressed sidelobes, in agreement with both simulations and SFG measurements. (d) Schematic of single-photon spectral measurement using calibrated tunable filters. (e) Independently measured single-photon spectra acquired via calibrated tunable filters, confirming the accuracy of the dispersion-based spectral measurement.
• Figure 4: Second-order correlation measurements of the signal/idler photons. (a) Schematic of measuring the unheralded second-order correlation function $g^{(2)}$ using a Hanbury--Brown--Twiss (HBT) interferometer. (b) Measured coincidence histogram and estimated purity of the signal photons, $P_\text{signal} = 82 \pm 2\%$. (c) Measured coincidence histogram and estimated purity of the idler photons, $P_\text{idler} = 89 \pm 3\%$.